1. Field of the Invention
The invention relates generally to management of logical volumes in a storage system.
2. Related Patents
This patent application is related to the following commonly owned United States Patent Applications, all filed on the same date herewith and all of which are herein incorporated by reference:
U.S. patent application Ser. No. 11-1500, entitled METHODS AND STRUCTURE FOR TASK MANAGEMENT IN STORAGE CONTROLLERS OF A CLUSTERED STORAGE SYSTEM;
U.S. patent application Ser. No. 11-1409, entitled METHODS AND STRUCTURE FOR DIRECT PASS THROUGH OF SHIPPED REQUESTS IN FAST PATH CIRCUITS OF A STORAGE CONTROLLER IN A CLUSTERED STORAGE SYSTEM;
U.S. patent application Ser. No. 11-1444, entitled METHODS AND STRUCTURE FOR LOAD BALANCING OF BACKGROUND TASKS BETWEEN STORAGE CONTROLLERS IN A CLUSTERED STORAGE ENVIRONMENT;
U.S. patent application Ser. No. 11-1484, entitled METHODS AND STRUCTURE FOR TRANSFERRING OWNERSHIP OF A LOGICAL VOLUME BY TRANSFER OF NATIVE-FORMAT METADATA IN A CLUSTERED STORAGE ENVIRONMENT;
U.S. patent application Ser. No. 11-1502, entitled METHODS AND STRUCTURE FOR IMPROVED I/O SHIPPING IN A CLUSTERED STORAGE SYSTEM;
U.S. patent application Ser. No. 11-1504, entitled METHODS AND STRUCTURE FOR MANAGING VISIBILITY OF DEVICES IN A CLUSTERED STORAGE SYSTEM; and
U.S. patent application Ser. No. 11-1557, entitled METHODS AND STRUCTURE FOR RESUMING BACKGROUND TASKS IN A CLUSTERED STORAGE ENVIRONMENT.
3. Discussion of Related Art
In the field of data storage, customers demand highly resilient data storage systems that also exhibit fast error recovery times. One type of storage system used to provide both of these characteristics is known as a clustered storage system.
A clustered storage system typically comprises a number of storage controllers, where each storage controller processes host Input/Output (I/O) requests directed to one or more logical volumes. The logical volumes reside on portions of one or more storage devices (e.g., hard disks) coupled with the storage controllers. Often, the logical volumes are configured as Redundant Array of Independent Disks (RAID) volumes in order to ensure an enhanced level of data integrity and/or performance.
A notable feature of clustered storage environments is that the storage controllers are capable of coordinating processing of host requests (e.g., by shipping I/O processing between each other) in order to enhance the performance of the storage environment. This includes intentionally transferring ownership of a logical volume from one storage controller to another. For example, a first storage controller may detect that it is currently undergoing a heavy processing load, and may assign ownership of a given logical volume to a second storage controller that has a smaller processing burden in order to increase the overall speed of the clustered storage system in handling I/O requests. Other storage controllers may then update information identifying which storage controller presently owns each logical volume. Thus, when an I/O request is received at a storage controller that does not own the logical volume identified in the request, the storage controller may “ship” the request to the storage controller that presently owns the identified logical volume.
FIG. 1 is a block diagram illustrating an example of a prior art clustered storage system 150. Clustered storage system 150 is indicated by the dashed box, and includes storage controllers 120, switched fabric 130, and logical volumes 140. Note that a “clustered storage system” (as used herein) does not necessarily include host systems and associated functionality (e.g., hosts, application-layer services, operating systems, clustered computing nodes, etc.). However, storage controllers 120 and hosts 110 may be tightly integrated physically. For example, storage controllers 120 may comprise Host Bus Adapters (HBA's) coupled with a corresponding host 110 through a peripheral bus structure of host 110. According to FIG. 1, hosts 110 provide I/O requests to storage controllers 120 of clustered storage system 150. Storage controllers 120 are coupled via switched fabric 130 (e.g., a Serial Attached SCSI (SAS) fabric or any other suitable communication medium and protocol) for communication with each other and with a number of storage devices 142 on which logical volumes 140 are stored.
FIG. 2 is a block diagram illustrating another example of a prior art clustered storage system 250. In this example, clustered storage system 250 processes I/O requests from hosts 210 received via switched fabric 230. Storage controllers 220 are coupled for communication with storage devices 242 via switched fabric 235, which may be integral with or distinct from switched fabric 230. Storage devices 242 implement logical volumes 240. Many other configurations of hosts, storage controllers, switched fabric, and logical volumes are possible for clustered storage systems as a matter of design choice. Further, in many high reliability storage systems, all the depicted couplings may be duplicated for redundancy. Additionally, the interconnect fabrics may also be duplicated for redundancy.
While clustered storage systems provide a number of performance benefits over more traditional storage systems described above, problems may arise when storage controllers share access to a logical volume. Logical volumes are typically identified to host systems and storage controllers using a Logical Unit Number (LUN). A LUN provides a reference to the logical volume for I/O requests. In a clustered storage system, typically one storage controller has “ownership” of a logical volume. The storage controller generates a local LUN to identify the logical volumes that the storage controller owns to its associated host system. When logical volumes are shared, each storage controller that “sees” the logical volume may utilize a different LUN to identify the shared logical volume to their respective host systems. This can be problematic for a number of reasons.
The first reason that this can be problematic is that utilizing different local LUNs to access the logical volume can cause confusion in users. For example, host system A may reference a logical volume as LUN-x, while host system B may reference the same logical volume as LUN-y. Thus, when a user changes between using host system A and using host system B, the same logical volume is referenced differently.
The second reason that the use of different local LUNs to access the same logical volume can be problematic is due to I/O shipping. As discussed above with respect to FIGS. 1 and 2, I/O requests in a clustered storage system may be shipped from storage controllers that do not own a logical volume to storage controllers that own the logical volume. When I/O shipping to a shared logical volume occurs, storage controllers may perform a number of LUN translations and LUN analysis as the I/O requests are shipped from one storage controller to another storage controller. This may occur because different storage controllers expose the shared logical volume to their respective host systems using different LUNs. In continuing with the example, consider that storage controller A is coupled with host system A. Storage controller A references the shared logical volume as LUN-x. In this example, storage controller A does not own the shared logical volume. Instead, the shared logical volume is owned by storage controller B that is coupled with host system B. Storage controller B references the shared volume as LUN-y. In this case, I/O requests generated by host system A use LUN-x as the target LUN, which are sent to storage controller A. Storage controller A is then tasked with analyzing the I/O request to determine which controller “owns” LUN-y (controller B in the example), translating the I/O requests from target LUN-x to target LUN-y, and ships the I/O request to storage controller B. This places a computational burden on the storage controllers in a clustered storage system.
Thus it is an ongoing challenge to reduce the computational burden on storage controllers within a clustered storage system.